1. Field of the Invention
This disclosure relates generally to nacelles for aircraft turbofan engines and, more particularly, to a nacelle having a variable-area fan nozzle (VAFN) that moves during thrust reverser deployment.
2. Description of the Related Art
Typical aircraft turbofan jet engines include an engine core, a nacelle that surrounds the engine core, and a fan that draws in a flow of air that is split into a bypass airflow and an engine core airflow. The nacelle provides a bypass duct that surrounds the engine core. The bypass airflow is transported through the bypass duct. The nacelle is configured to promote laminar flow of air through the bypass duct. The engine core includes a multi-stage compressor to compress the engine core airflow, a combustor to add thermal energy to the compressed engine core airflow, and a turbine section downstream of the combustor to produce mechanical power from the engine core airflow. The typical turbine section has two and sometimes three turbine stages. The turbine stages are used to drive the compressor and the fan. After exiting from the turbine section, the engine core airflow exits the nacelle through an exhaust nozzle at the aft end of the engine.
Modern aircraft employ turbofan variants of gas turbine engines that have a low fan pressure ratio (FPR) and high bypass ratio (BPR) for a bypass duct of the engine. As turbofan engine technology has matured, designers have stretched the associated technologies to minimize weight, cost, and maintenance while maximizing efficiency. Because of the extreme limits to which these engines are designed, and the valuable commercial interest in having the most efficient product available, even incremental improvements are sought after in the design of commercial turbofan engines.
One approach for optimizing the performance of an engine over various flight conditions involves varying the fan nozzle exit area. By selectively varying the fan nozzle's exit area during flight, an engine's bypass flow characteristics can be adjusted to better match a particular flight condition, for example, by optimizing the FPR relative to the particular thrust level being employed. For example, a variable area fan nozzle (VAFN) assembly that forms a rear outer portion of the bypass duct can include an airfoil that is moved aft into a VAFN flow position so as to open an additional bypass flow that exits the nacelle forward of the VAFN assembly. That is, an opening is created between the translatable sleeve and the VAFN airfoil, such that an airflow in the bypass duct is split into a first flow portion that remains in the bypass duct and moves past the airfoil, and a second flow portion that exits the bypass duct through the VAFN opening and over an outer surface of the airfoil. Optimum performance is achieved when the VAFN is properly aligned with the remainder of the nacelle throughout all flight regimes. In other words, the VAFN should not be skewed vis-á-vis the nacelle during flight.
In a turbofan engine, the fan typically produces a majority of the thrust produced by the engine. The bypass airflow can be used to produce reverse thrust, typically employed during landing. A thrust reversing apparatus that forms a portion of the nacelle selectively reverses the direction of the bypass airflow to generate reverse thrust. During normal engine operation, the bypass airflow may or may not be mixed with the engine core airflow exhaust prior to exiting the engine nacelle assembly. During thrust reverse operation, a translatable sleeve is moved from a stowed position to a deployed position to expose a cascade assembly, and blocker doors are deployed into the bypass duct. In this deployed position, the blocker doors redirect the airflow in the bypass duct to exit the nacelle out the cascade assembly.
Some VAFN systems function by adjusting the VAFN using actuators that are independent of the thrust reverser sleeve actuators. The trailing edge of a translating sleeve on a cascade-type thrust reverser is a space-limited environment within the nacelle, especially when used with a VAFN. Because of space limitations and the difficulty of running wires and associated mechanisms, actuators for adjusting the VAFN are typically mounted on nacelle non-moving structure forward of the thrust reverser.
There is a need in the art for lighter, more efficient, and more reliable engine components including VAFN assemblies.